Isle of Dread
Isle of Dread was the first published Adventure for any version of Dungeons & Dragons to focus on wilderness exploration as a major theme. This would go on to be an important element in many other adventures, including most of the rest of the X series. It also introduced numerous creatures to the game for the first time, including the kopru and aranea, the rakasta and phanaton, and many others, including several types of dinosaurs.
The adventure came with a fairly detailed (for the time) map of a setting then called the "Known World", showing no less than 15 distinct nations on the mainland to the north, as well as much of the Sea of Dread in which the Isle of Dread could be found. These nations each received a paragraph or so of description near the beginning of the module. This, along with a few pages in the Expert Rulebook on one of these nations, was the first significant information to be made available on the world that was later known as Mystara.
Isle of Dread was ranked the 16th greatest Dungeons & Dragons adventure of all time by Dungeon magazine in 2004, on the 30th anniversary of the Dungeons & Dragons game.